The xeroprinting process employs a printing plate, commonly referred to as a "master", made by creating a pattern of insulating material (i.e., an image) on the surface of a grounded conductive substrate. In the xeroprinting process, an electrostatic charge is applied to the surface of the master, e.g., by corona discharge. The portion of the master bearing the insulating material retains the charge, while the charge on the remainder of the master is discharged through the grounded conductive substrate. Thus, a latent image of electrostatic charge is formed on the insulating material, the image subsequently being developed with either oppositely charged particles commonly referred to as "toner" or liquid electrostatic developers. The toner is then transferred (e.g., by electrostatic or other means) to another surface (e.g., paper or polymeric film), where it is fused (i.e., "fixed"), to reproduce the image of the master. Since the image on the master is permanent, or at least persistent, multiple copies can be made by repeating the charging, toning and transfer steps.
Recently issued U.S. Pat. No. 4,732,831 to Riesenfeld et al. discloses an improved xeroprinting process that employs a master having a photopolymerizable coating on a conducting substrate. The coating contains an organic polymeric binder, an ethylenically unsaturated monomer, and a photoinitiator system. When the master is exposed to the desired pattern of actinic radiation (i.e., light of a suitable wavelength), exposed regions of the coating polymerize and exhibit a significantly higher electrical resistance than unexposed regions. Thus, when the master is subsequently used in the xeroprinting process, the polymerized regions will hold an electrical charge, which is developed with toner, while the unpolymerized regions discharge to ground through the conductive backing and therefore do not attract the toner.
It has been found that the electrostatic properties of photopolymerizable masters change considerably with small variations in ambient temperature around room temperature (RT). Relatively small changes in humidity at these temperature conditions also affects electrostatic properties. For example, the discharge rates of the photopolymerizable layer increase with a rise in temperature. Changes in the discharge rate with ambient temperature result in degradation of print quality as well as unacceptable dot gain and dot range. Lower temperatures (RT-5.degree. C.) show lack of shadow dots while at higher temperatures (RT+5.degree. C.) highlight dots and dot gains diminish.
There has been no attempt to understand the change in resistivity upon exposure of a photopolymerizable element or the details of the conduction mechanism. Transport of charge in photopolymerizable elements, however, has been studied. The photopolymerizable electrostatic master when unexposed has its glass transition temperature (Tg) near ambient temperature. The electrical conductivity is the result of conduction by impurity ions with a reasonable degree of mobility in the liquid-like photopolymerizable layer of high viscosity. Upon imagewise exposure, the Tg of the photopolymerized layer shifts closer to the Tg of the binder component, e.g., about 100.degree. C., and the i photopolymerized areas are in a glassy state. The electrical conductivity decreases several orders of magnitude corresponding to the change in charge mobility in the photopolymerized (glassy) areas. The modification of the electrical properties of the photopolymerizable electrostatic master by change in glass transitions made one having ordinary skill in the art believe that the change in Tg affected the temperature and humidity sensitivity of the master as well. Based on this assumption it appeared unlikely that photopolymerizable electrostatic masters would be achieved that were substantially insensitive to the changes in the environment, e.g., temperature and humidity.
It has now been found that a photopolymerizable electrostatic master having improved environmental latitude can be made wherein the above disadvantages are substantially overcome by introducing into the photopolymerizable composition forming the photopolymerizable layer a blend of binders, at least one binder having a relatively higher glass transition temperature than at least one other binder present. The improved photopolymerizable electrostatic master exhibits good image quality, electrical properties and temperature stability.